Cartridge type electrical fuses having axial leads have been long known in the fuse art. The fuse element in such a fuse is typically a fusible wire centrally supported within a cylindrical sleeve forming a casing for the fuse. To insure reliable fusing it is essential that the fuse wire must not touch the interior wall of the sleeve along its length, hence, the ends of the fuse wire are supported in such a manner as to prevent such contact. External lead carrying end caps having solder therein are used to capture the fuse wire ends folded over the outside of the sleeve ends. Final mechanical assembly consists of press fitting the end caps over the folded-over ends of the fuse wire followed by momentary heating of the solder to obtain good electrical connection between the fuse wire and the end caps. Since the fuse casing formed by the sleeve must form an insulated body, typically made of ceramic or glass, which cannot be solder bonded, the only substantial opposition to the separation of the end caps from the sleeve is derived from the pressure fitting of the end caps over the outer surface of the sleeve. Thus, such fuse structures are generally weak in tension, and are prone to mechanical failure on a pull test applied to the end leads. The alternative construction is to solder bond the end caps to the sleeve ends, which requires an expensive local outer metallization of the sleeve ends. Such structures are prone to humidity induced corrosion problems because of the exposed metal end caps and the lack of any hermetic sealing thereof.
One prior art partial solution to the abovementioned problems comprises the application of a length of heat-shrinkable plastic tubing tightly heat shrunk over the sleeve and end caps, the tubing overlapping, although loosely, the inner end of the leads extending outwardly from the end caps. The heat shrunk tubing provides some improvement in fuse strength and provides a moderately good sealing for the fuse interior. A disadvantage of this construction is that the cap ends are exposed to the external ambient conditions, owing to the fact that the limited shrinkage capability of the tubing prevents a desired end cap sealing engagement of the heat shrunk tubing with the leads. Thus, it is still necessary to plate the end caps to secure adequate corrosion resistance for these elements. One of the objects of this invention is to eliminate the necessity of anticorrosion plating of the end caps.
The resulting structure is still not adequately strong, in that a moderate pull on the leads can still shift the end caps to break the fuse wire. A further object of this invention is to improve the structural strength of the fused structure, particularly with respect to lead pull.
Additionally, in small fuses of the order of eight millimeter length or less, the end caps typically are of significantly larger diameter than the fuse sleeve. Since heat shrink tubing closely follows the outer contours of the fuse structure, a marked step in the fuse profile remains in the region of the end cap to sleeve junction. In fuses of small length, color coding bands indicating the fuse characteristics must necessarily be disposed along substantially the entire length of the fuse structure to be distinctly visible. This in turn typically necessitates that one or more of the coding bands must overlie a step. In practice this proves to be uneconomical if conventional color wheels are used to apply the bands, since very erratic striping results in the vicinity of the step, leading in turn to unacceptably low production yields.
One not completely satisfactory solution to this problem has been to print the fuse amperage in alphanumeric characters on the fuse body and to use clear transparent shrink tubing through which such indicia can be read. In the case of very small fuses, such numbers are difficult to read with the naked eye. A further object of this invention is to provide a fuse that can be inexpensively and reliably coded on the outside thereof by means of readily visible color bands.